WASHINGTON — Death rates from cancer continue to inch down, researchers reported Monday. Now the question is how to hold onto those gains, and do even better, even as the population gets older and fatter, both risks for developing cancer. Overall, deaths from cancer began slowly dropping in the 1990s, and Monday’s report shows the trend holding. Among men, cancer death rates dropped by 1.8 percent a year between 2000 and 2009, and by 1.4 percent a year among women. The drops are thanks mostly to gains against some of the leading types — lung, colorectal, breast and prostate cancers — because of treatment advances and better screening. “There has been clear progress,” said Dr. Otis Brawley of the American Cancer Society, which compiled the annual cancer report with government and cancer advocacy groups. But bad diets, lack of physical activity and obesity together wield “incredible forces against this decline in mortality,” Brawley said. He warned that over the next decade, that trio could surpass tobacco as the leading cause of cancer in the U.S. Deaths still are rising for certain cancer types including liver, pancreatic and, among men, melanoma, the most serious kind of skin cancer.